An optical fiber is produced by drawing a preform on a drawing tower. A preform generally includes a primary preform consisting of a very high quality glass tube that forms a part of the cladding and the fiber core. This primary preform is then overcladded (or sleeved) to increase its diameter and form a preform that can be used on a drawing tower.
The primary preform is then overcladded using silica particles to yield a final preform. Although natural silica particles are typically used, synthetic and/or doped silica particles, which are relatively more expensive, may also be used. The overcladding of the primary preform may be conducted by plasma deposition during which particles of natural silica are projected and fused by a plasma torch at a temperature near 2,300° C. At this elevated temperature, the natural silica particles vitrify on the periphery of the primary preform. The overcladding operation is generally conducted in a closed chamber under a controlled atmosphere to ensure protection against electromagnetic disturbances and the release of ozone that is emitted by the plasma torch.
FIG. 1 schematically depicts a cross-sectional view of a conventional plasma torch 200 for overcladding an optical fiber preform 100. One such conventional plasma torch 200 is disclosed, for example, in European Patent Publication No. EP 1213950 A2 or U.S. Patent Application Publication No. 2003/0182971.
U.S. Pat. No. 4,833,294 discloses a plasma torch that includes a plasma tube for confining and directing gas flows within an electromagnetic field produced by a coil. The torch further includes a base member to receive the plasma tube and an exteriorly threaded insert member.
The plasma torch 200 depicted in FIG. 1 includes a confinement tube 201, which is used for confining the plasma. The confinement tube 201 may have a multi-wall structure to allow for flow of a liquid coolant. The confinement tube 201 of the plasma torch 200 should be capable of withstanding the extremely high temperatures that are generated in the region of the plasma. The confinement tube 201 is thus generally quartz, but it may also be thermo-conductive ceramic such as described in U.S. Pat. No. 5,200,595.
The plasma torch 200 depicted in FIG. 1 also includes a torch base 500, which is attached to one end of the confinement tube 201. The torch base 500 itself includes a support 502 and a gas diffuser 400. The support 502 and the gas diffuser 400 are generally stainless steel. At least one main gas inlet 203 is provided for injecting pressurized air into the confinement tube 201 of the plasma torch 200 in order to feed the plasma. An initiator gas, such as argon, may be injected at the beginning of the operation of the plasma torch 200 because of the low capability of air to initiate ionization. An induction coil 202 is wound around the confinement tube 201. The induction coil 202 is powered by the induction generator 210. Alternating electric current generates an electromagnetic field, which ionizes the gas (e.g., air) in the confinement tube 201 in order to create a plasma flame 600.
FIG. 1 also depicts an optical fiber preform 100 and projected silica grains 1000 between the preform 100 and the plasma flame 600. The silica grains 1000 are projected from a projection tube 300, which may optionally be integrated to the plasma torch 200.
The induction generator 210 used with the plasma torch 200 typically provides a maximum power on the order of 200 kilowatts (kW), but the power which may be applied to the induction coil 202 of the plasma torch 200 is often limited to about 100 kilowatts because of the conventional design of the plasma torch 200.
The greater the power of the induction generator 210, the larger the flame 600 of the plasma and the faster the overcladding may be carried out (i.e., more silica grains 1000 may be vitrified per unit of time). Therefore, for reasons of productivity and yield, it is desirable to increase the power of the plasma induced in the plasma torch 200 to, for example, 130-150 kilowatts.
When the power of the induction generator 210 increases, the plasma flame 600 extends. This extension towards the outside of the confinement tube 201 of the plasma torch 200 is beneficial because the plasma flame 600 then includes a larger amount of projected silica grains 1000 in front of the preform 100 and the overcladding yield is improved. When the plasma flame 600 is extended, however, it is also extended inside the confinement tube 201 and approaches the torch base 500. The torch base 500 then undergoes strong thermal stresses that may damage it. For example, when the power of the induction generator 210 of the plasma torch 200 has been increased in this way, the inventors have observed the occurrence of burning (e.g., charring or blackening) and flaking in the steel torch bases 500. Such deterioration of the torch base 500 may lead to the projection of impurities in overcladding the preform 100. Such contamination requires that the preform 100 be discarded, resulting in a productivity loss.
Therefore, there is a need for a plasma torch base that may withstand stronger operating power without deteriorating.